1. Field of the Invention
The present invention relates generally to lasers, and in particular, to characterizing the quality of laser beams.
2. Related Art
Laser beams are used today in a wide range of applications, ranging from eye surgery to the manufacture of semiconductor chips. The various applications typically require different types of lasers, such as solid state lasers, gas lasers, excimer lasers, dye lasers, and semiconductor or diode lasers. Also, depending on the type of laser and/or cavity, the generated beam can be classified by the type or profile, such as top hat, Gaussian, super Gaussian, and other transverse modes or combinations of transverse modes.
Another descriptor of laser beams is beam quality. In many applications, it is important to know something about the laser beam quality, i.e., how the beam departs from a theoretical perfect beam. The beam quality affects how the beam will propagate, as well as how tightly it will focus. Beam quality can worsen from a variety of factors, including misalignment, optics degradation, and laser deterioration. Therefore, with applications in which one needs to know how well the laser can focus the beam in a small interaction area, a measure of beam quality is desirable.
Unfortunately, it is often difficult to obtain a measure of beam quality, as evidenced in part by the numerous methods of determining beam quality. Some known methods include the M2 parameter (space-beamwidth product), Strehl ratio, root-mean-squared (RMS) wavefront error or interferometry, and power-in-the-bucket (PIB). These methods all have advantages in some situations and deficiencies in other situations, such as the type of beam to be measured.
For example, the M2 parameter has become a commonly used parameter to generally describe near-Gaussian laser beams. Most methods involve obtaining M2 by measuring propagation distributions at multiple locations along the beam path. The M2 parameter is especially useful in that it allows a prediction of the real beam spot size and average irradiance at any successive plane using simple analytic expressions. This provides system designers with the ability to know critical beam parameters at arbitrary planes in the optical system. However, when the laser beam shows any vignetting effects, e.g., from finite apertures, the M2 measurement is not effective. Similarly, for other methods, such as in wavefront interferometry, a good coherence length and stabilized pointing is needed for proper determination of beam quality. Thus, present beam quality measurements are incomplete and inconsistent.
Accordingly, there is a need for a system and method of measuring the quality of laser beams which is suitable for many different types of beams.